At some point in your homebrewing career, you are likely to take a gravity reading during a brew session, and think, "what the heck went wrong?" If you can be bothered, you search through the vast halls of the university of homebrewing that lives on the Internet. Then you land upon the term “brewhouse efficiency”. This may appear in an article, blog post, recipe, or even your favorite brewing software that is offering to calculate something for you. To really tackle this question I want to first break down the phrase into its two components individually, and take a moment to better understand them.
The term brewhouse usually is intended to refer to the equipment used to produce wort. This extends beyond just the mash; it includes all pumps, hot liquor tanks, boil kettles, grants or anything else you might use to produce the wort. Because losses can occur at any point in the wort production, calculating the amount of gravity obtained in the final volume, post boil, is the main idea behind calculating brewhouse efficiency. Your brewhouse is all the equipment used to make and wort.
The general definition of how efficiency is measured, no matter the context, comes down to approximating the best estimate possible of how much return we get from the energy or cost put into the producing of a certain product or result. It is usually represented by a percentage, with higher values indicating greater output. In other words, you might say it measures "how much bang for your buck". In a brewing setting, when comparing two batches, a higher efficiency in one batch might indicate a greater yield per unit of grain. All else being equal, you might have extracted more fermentable sugars (and have a higher starting gravity) with the same amount of grain as in the other batch. Or, you might obtain an equal gravity measurement between two batches, but would have used less grain in one of the batches, also yielding a higher efficiency in the batch with less grain than in the batch with more grain. In both of these scenarios, your overall efficiency in the brewhouse would be a measurable number, but you might not always know how to identify where in your process that greater output was achieved.
Things that affect efficiency are the quality of your grain crush, better temperature or PH management, or might have done a number of other things to help improve the extraction of fermentable sugars from your grains and with the desired post-boil volume. The key is to maintain copious notes about your methods, equipment and ingredient selection that all could have impacted the overall effects of your brew, and whether you hit your target gravity and target volume. If you are working to improve your efficiency, change one thing each brew-day so you can better track how it affects your numbers. An important thing to keep in mind is that lower expected efficiency usually means you will need more grain to hit your target gravity than a brewhouse higher overall efficiency, unless there are improvements that can be made to your equipment and/or processes. A poor grain crush, bad PH and temperature management all can lead to a lower than expected efficiency
The actual formula for efficiency usually looks something like "input/output". To begin determining your brewhouse efficiency for a given batch, start with calculating the total potential gravity to be obtained from all grain in the recipe:
Grain gravity points: is usually acquired, by using only the last two digits of the potential gravity of a grain. For example, if a pale malt has a potential gravity of 1.036, its grain gravity points are taken as 36 for this calculation. The weight and volume units can be metric or imperial units, but must be kept consistent in both the numerator and denominator. When we add up all of the potential gravity points from all the fermentable grains and adjuncts in the recipe, we obtain the total expected potential gravity points. However, this is assuming 100% efficiency. In reality, a brewing setup would yield somewhere between 70%-80% efficiency. Once you have a starting gravity measurement (the reading you take at yeast pitch), and a calculation of total potential gravity points
For example, let's say our expected gravity (at 100% efficiency) = 1.045, or 45 points. However we measured a gravity of 1.038 into the fermenter. Then our efficiency would be = 38/45 = roughly 84%.
Let's say you end up a gallon short on your expected final volume, and are still below your target gravity despite the fact (not good!). It's okay if your volume is low but your gravity is higher than expected because you can add water to increase the volume and lower the gravity. For example, suppose I was targeting volume of 5 gallons with a total potential gravity of 57 points, but only yielded the 1.040 at 4 gallons. Our overall efficiency in this case would be
Which in this case would yield (40 * 4) / (57 * 5) = 0.711 or 56%
If you really need to hit your volume and your numbers fell short, you can add a bit of extract to get your recipe back on track. This is why it's always nice to have some extract on hand, whether it's for making starters or brewhouse efficiency mishaps. If you'd like a consistent PH without getting into water chemistry too much, try 5.2 PH Stabilizer, which helps maintain a stable PH from batch to batch. Good luck, and happy brewing!
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